When viewing a film, it is often desirable to skip over uninteresting scenes or, conversely, to rewind the film to repeat certain scenes. As a result, virtually all video playback units include fast-forward and rewind controls that enable the viewer to rapidly move forward or backward along the film.
However, without the ability to identify selected portions of the film, it is difficult for a viewer to determine how long to operate in fast-forward or rewind mode. To address this difficulty, virtually all video playback units provide some position-indicating feedback to the viewer. A particularly useful method of providing such feedback is to continue displaying the film when operating in fast-forward or rewind. These two types of displays are collectively referred to in the industry as “trick-mode” displays.
In both analog and digital video delivery systems, an ordered sequence of images is shown to the viewer at a rate (approximately 24 images per second) that is fast enough to give the user the illusion of motion. Aside from the improved image and sound quality associated with digital video, there is little noticeable difference between these delivery systems so long as they operate in normal mode. The difference between analog and digital video delivery systems becomes quite apparent, however, when one switches to trick-mode display.
When operating in trick-mode, an analog video delivery system, such as a video tape recorder, simply speeds up the rate at which the medium containing the video signal slides past a read head. To a first approximation, this results in a uniform compression of the temporal axis. A viewer thus sees all the action in the film being performed at a uniformly accelerated pace.
In contrast, a digital video delivery system operating in trick-mode generally does not show each image from the sequence of images making up the film. Instead, a trick-mode processor selects a subset of images from the film and transmits those images to a decoder for display to the viewer. Since these selected images are generally represented by differing amounts of data, they take varying amounts of time to reach the decoder and varying amounts of time to be processed by the decoder. The sum of the transmission time and the processing time is referred to as the “delivery interval.”
When the decoder receives a first selected image, it decodes it and provides the resulting signal to the video input of a television for display to the viewer. The decoder repeatedly provides this signal to the video input until a second selected image becomes available for display. The viewer thus sees the first selected image while the decoder processes the second selected image. When the decoder completes processing the second selected image, it provides this new signal to the video input. The viewer then sees the second selected image.
The length of the time interval during which the viewer sees the first selected image thus depends on the time required to have the second selected image ready for display. Since the selected images can have very different sizes, this time interval can vary significantly. For example, if the second image is represented using only a very small amount of data, only a short time elapses before it is ready for display. Consequently, the viewer will see the first image for only a very short time before it is replaced by the second image. Conversely, if the second image requires considerable data for representation, a long time elapses before it is ready for display. Consequently, the viewer will see the first image for an extended period before it is finally replaced by the second image.
A digital video delivery system operating in trick-mode thus displays selected images for varying amounts of time. As a result, a viewer who activates trick-mode for a fixed number of seconds will advance or rewind the film by unpredictable amounts of time. This makes it difficult to judge, by watching the sequence of images go by, how much time has elapsed in the film. In addition, the subjective experience of watching a sequence of images in which each image is displayed for a seemingly random time can be unpleasant.